We are investigating the role of intracellular Na ion on the regulation of Na ion transport across the rabbit urinary bladder. Preliminary findings show a high correlation between net transport and (Na)i with (Na)i being low in highly transporting tissues and high in poorly transporting ones. There is also a strong negative feedback effect of intracellular Na ion on the conductance of the apical membrane. This feedback control represents the rate limiting step in the Na ion transport process. Additionally, we are also studying the effects of nucleotides and pH on the bladder.